X-ray image diagnosis apparatuses are apparatuses which obtain an X-ray image of a subject such as a patient by irradiating the subject with X rays by means of an X-ray irradiator and detecting the X rays having passed through the subject by means of an X-ray detector. As these X-ray image diagnosis apparatuses, an X-ray image diagnosis apparatus has been developed which, for example, includes a C arm configured to hold the X-ray irradiator and the X-ray detector in a mutually facing state and the like, moves the X-ray irradiator and the X-ray detector to an imaging position relative to the subject on a top panel to capture an X-ray image of a target part of the subject, and displays the X-ray image on a monitor.
Such an X-ray image diagnosis apparatus operates in a normal imaging mode and also in an X-ray irradiation mode which may be called a fluoroscopy mode that involves continuous application of a small amount of X rays from the X-ray irradiator and continuous display of X-ray images of the subject. The fluoroscopy mode has often been utilized to locate a target part or to observe a moving part in a moving image. Using the fluoroscopy mode can reduce the amount of X-ray irradiation of the subject and therefore reduce the exposure dose of the subject. Note that X-ray images captured in the fluoroscopy mode are generally called fluoroscopic images.
While reduction in the exposure of the subject is achieved as described above, further reduction in the exposure is desired. In this respect, techniques (e.g. spot imaging method and the like) have been proposed which involve: setting a region of interest (ROI) in a fluoroscopic image (still image) stored in advance; irradiating only the set region of interest with X rays to obtain fluoroscopic images (moving image) of the region of interest; and superimposing the moving image on the still image and displaying them. These techniques have been used, for example, when ablation using a catheter is performed, or in some other similar situation.
However, in the techniques that create a synthesized image by superimposing a moving image on a still image as described above, there is a time difference between the still image stored in advance and the moving image in terms of the timing to capture the image. Thus, the positional relationship of the subject relative to the still image and that relative to the moving image may become different upon body movement of the patient, or the subject, or the like. This may possibly make the synthesized image invalid. To solve this, the region of interest is changed in response to movement of the subject. This, however, requires the still image stored in advance to be changed at the same time. Without doing so, the positional relationship between the region of interest and its peripheral region becomes inaccurate, thereby making it impossible to accurately figure out their positional relationship. Moreover, changing the still image stored in advance requires re-capturing a fluoroscopic image of the peripheral region. This increases the exposure dose and also deteriorates the operability.